Superconductors, in particular ceramic high-temperature superconductors, offer a great potential as fault current limiters which enable rapid and effective current limitation, automatic recovery, negligible impedance during normal operation and application at high voltage.
Current limiters based on high-temperature superconducting materials make use of the property of superconducting materials to switch from their superconducting state with essentially no resistance to a normal conductive state when at least one of its critical temperature (Tc), critical magnetic field (Hc) or critical current (Ic) is exceeded.
For example, in case of fault the current flowing through the superconductor material exceeds the critical current of the superconductor material due to large surge current and the superconducting material undergoes transition from the superconducting state to the normal conducting state. This transition is also termed “quenching”. For good operation, after the current limiting event the superconductor should have the capability to return to its superconducting state.
There are known different embodiments of superconducting current limiters. In so called resistive (ohmic) limiters a superconducting element becomes normally conductive. The occurring resistance limits the current in the case of fault current events.
There are also known so called inductive current limiters. When applied with fault current, the inductive current limiter provides a high impedance which limits the fault current below a threshold level. For example, U.S. Pat. No. 5,140,290 discloses a device for inductive current limiting of alternating current, in which the current to be limited flows through an induction coil made of copper. A hollow cylinder of a high temperature superconducting material is arranged in the interior of this coil, and a soft magnetic material with high permeability is arranged concentrically inside. In normal operation (rated current) the superconductivity of the hollow cylinder shields the magnetic field of the induction coil completely from the core and impedance of the induction coil is maintained at a very low level. When a fault current flows through the induction coil the superconductivity of the hollow cylinder body disappears and the impedance of the induction coil reaches its maximum current-limiting value.
In all the different limiter concepts, due to the increasing resistance during quench the superconducting material is heated up along its length.
In practical, however, the superconducting material of which a superconductor body is made is not completely homogeneous throughout the superconductor body, so that the superconductor properties such as critical current density can be different at different regions of the superconductor body. Consequently, in case of fault current some regions become already resistive whereas other regions still remain superconducting. Due to the still superconducting regions high current flows through the superconductor body leading to a high temperature increase in the already resistive regions and causing burn out in these regions. Thus, in order to avoid damage of the superconductor body during quenching it is necessary that the quenching and, consequently heating of the superconductor body, occurs as homogenously and rapidly as possible so that the superconductor body becomes resistive as a whole within a time sufficiently short to avoid thermal destruction of the superconductor body due to local overheating (“hot spots”).
For solving the problem of local overheating it was known to provide the superconductor body with a parallel shunt of normal conducting material whose resistance is lower than the resistance of the superconducting material in its normal conducting state. In case of default and, thus, sudden temperature increase, current is by-passed to the shunt and heat is dissipated. In the course thereof the shunt material is heated up.
Commonly, the electrically conducting material for the shunt is a metal such as copper or copper alloy.
For example, resistive current limiters are suggested which are composed of a cylindrical tubular superconductor body having a stripe of electrically conducting metal coated on its outer surface in parallel to the longitudinal axis and reaching from one end of the cylindrical tubular body to its other end. Superconductor components related to such configuration are referred to in WO 00/8657.
In DE 42 34 312 a resistive high-temperature superconductor current limiter is disclosed wherein the superconductor component has the shape of a bifilar spiral. On one face of the bifilar spiral made of the superconductor material all along its winding a metal coating is applied for the shunt.
Since in all of these configurations the shunts are arranged in parallel with the superconductor body both the shunt as well as the superconductor body have nearly the same length.
The necessary minimum length of the current limiter at a given voltage, however, is defined by the maximum strength of the electrical field which can be applied along the normal conducting shunt during short circuit. If the electrical field acting on the shunt during short circuit exceeds the maximum value overheating of the shunt occurs with temperatures exceeding the melting point of the shunt material and, consequently, the shunt melts.
For illustration, the maximum electrical field applicable to commonly used shunt materials is about 1 V/cm. That is, if a voltage of 10 kV is applied to the current limiter the length of the superconductor body has to be about 100 m in order to limit the electrical field to which the shunt is subjected during fault event to about 1 V/cm.
However, this means, that, for example, in high-voltage applications high-temperature superconductor bodies are required having a very long length which in turn requires large material need with subsequent costs.
It was also known to apply an external magnetic field to the superconductor body of a superconducting current limiting device in order to assist quenching. By said magnetic field the critical current density of the superconductor material is reduced which, in turn, promotes quenching. These devices make use of the fact, that the critical current density decreases with increasing magnetic field.
In U.S. Pat. No. 6,043,731 a superconducting current limiting device is disclosed wherein magnetic field generating means are provided for generating and applying, during normal operation in the superconducting state, a magnetic field to the superconductor element in order to hold the current density below the critical current density. In the fault event the magnetic field is adjusted, that is, increased to reduce the critical current density in order to bring the superconductor element to its resistive state.
According to one embodiment of U.S. Pat. No. 6,043,731 the superconductor element is positioned within a shunt coil which is connected in parallel to the superconductor element. In fault event excess current is forced into the shunt coil and the current flow in the shunt coil generates a magnetic field which acts on the superconductor element and decreases the critical current density, thereby assisting quenching.
As superconductor element a thin film superconductor is disclosed on a semiconducting substrate. According to the figure layers of superconductor thin films and layers of the substrate alternate, that is the superconductor element is a stack of layers of superconductor thin films and layers of substrates.
However, further details as to the specific constructions of this embodiment, in particular of the resulting shape of the superconductor element, are not given.
Furthermore, a recently favored geometry is disclosed in J. Bock, F. Breuer, H. Walter, M. Noe, R. Kreutz, M. Kleimaier, K. H. Weck, S. Elschner, “Development and successful testing of MCP-BSCCO 2212 components for a 10 MVA resistive fault current limiter”, Supercond. Sci. Technol. 17, pp. S122-S126, 2004. Here bifilar coils are used for the current limiters. Due to the bifilar geometry the induction inherent to coil configuration is small. However, since in bifilar coils input and exit for the current are close together, in particular with respect to high voltage applications insulation is problematical.